Light pens are used with cathode ray tube (CRT) displays as input-output devices for computers in a variety of applications. A light pen held against the video screen detects the flash of light from the scanning electron beam as it passes the pen. The signal from the light pen can be compared with the vertical and horizontal deflection signals of the CRT to determine the location of the pen at the time that the electron beam passed by it. This location information can then be used by a computer to provide an interactive graphics display on the screen. For example, the light pen user can issue a command to the computer by pointing to a particular position of the display on the screen.
Light pens are typically operable only when pressed against the face of the video screen so that light is received by the light pen from a very small portion of the screen. If the light pen is withdrawn from the screen, the intensity of light received by the light pen photodetector as the electron beam passes is reduced, the detector receives light impulses from several of the horizontal raster lines of the electron beam as it covers the field of the screen, and external ambient light sources, particularly light reflected off the glass face of the screen, will interfere with the detection of the image on the screen. The necessity of placing the light pen in physical contact with the screen presents no limitation for conventional light pen applications such as computer aided drawing, but it does prevent the use of a light pen when the user either cannot or should not physically touch the screen, or where it would be of advantage for the user not to be near the screen.
In particular, physically disabled people may not have the ability to use a conventional light pen to communicate with a computer. Many individuals have lost the use of their arms or hands because of accidents or illness. They are unable to write with pen and pencil, or to stroke the keys of a standard keyboard, and may similarly be incapable of using a light pen in the standard fashion to input data to a computer through a CRT screen. Because persons so disabled lack the ability to operate such standard tools of communication as computer or typewriter keyboards, many educational and vocational opportunities are foreclosed or at least extremely limited.
A number of specialized devices have been developed to aid the communication abilities of severely disabled individuals. Many of the severely disabled retain the ability to control head position and direct head pointing can thus be used as a data input technique. Devices which utilize head pointing as a data selection technique generally include a display unit and a pointer which is attached to the head. The display unit typically contains the alphabet and numeric digits, and may also include commonly used words or phrases. In one type of device in use today, a light emitting diode (LED) is mounted next to each vocabulary element on the display unit and each LED is turned on and off sequentially. A photodetector mounted to the user's head has a field of view substantially limited to the area occupied by a single LED. When the photodetector points toward the selection display, it detects the light emitted from one of the LEDs. The scanning is stopped and the LED detected remains lighted until the user points to a different position. If the user dwells at a position for a predetermined period of time, the vocabulary entry associated with the LED detected is selected and displayed or printed.
There are several problems with the LED head pointing system discussed above. A particular problem is the lack of continuous head position feedback, since the discrete LEDs only show the approximate area being pointed to on the selection display. The user may not know whether he is pointing directly at the LED or at an area just close enough to the LED to detect it. If the latter, the user can easily drift away from the desired position before the time required to cause selection of a vocabulary item has elapsed. In addition, the display itself is fixed in size, number of vocabulary items, and the labeling of the vocabulary items, preventing their adjustment by a user to suit individual needs. Because these displays are so specialized, and of little use outside the field of aids for the disabled, their cost is relatively high since the production and development costs must be spread over a relatively low production volume.
Devices employing other head pointing techniques have also been used. The simplest device may be a pointing rod or stick attached to the user's head which can be used to directly punch keys on a keyboard. Such a technique has obvious limitations, such as the speed with which the pointer can be moved, the awkwardness of manipulating the stick, the necessity for modifications to standard keyboards to allow shifting and control, and fatigue of the wearer because of the weight of the rod or stick. Light beam head pointer systems have also been developed which use a small flashlight, mounted to the user's head, which projects a narrow beam of light onto a communication board capable of detecting the spot of light. For example, a photoconductive array, with a photoconductor associated with each key on a keyboard, can be used in conjunction with the light beam pointer. Such photoconductive arrays have the disadvantage of being relatively expensive because of the custom, low volume nature of the keyboard and the complexity of the electronics required to decode the character or word pointed to by the individual. Problems are also encountered because of ambient light which may saturate the photodetectors, thereby preventing the user's light source from being detected or resulting in spurious detections.